Disk based data storage devices for storing digital electronic information have been in use in the computer industry for several decades. The storage devices operate by storing digital information on a disk media, such as magnetic or optical media. The disk media can be either rigid or flexible and is mounted on a rotating hub. The storage devices are commonly referred to as disk drives, and typically accommodate removable media or fixed media.
As disk diameters have become smaller, the issue of damage from mechanical shock has become a significant factor. Disk drives can be accidentally bumped, jarred, or dropped during operation. Because of their low weight, a disk drive can be subjected to a substantial amount of mechanical shock during normal operation of a computer.
One negative effect resulting from mechanical shock occurs when the drive is writing data to a disk. In this state, the head is positioned over the proper track to record the data. If the mechanical shock is severe enough to cause the head to move over an adjacent data track before the write current in the data head is turned off, the data in the adjacent track will be corrupted. This damaged data is not recoverable. Neither the computer nor the disk drive controller know what data was damaged, when and how it was originally generated, and has no way to fix the damaged data. The user will not even know data has been corrupted until a read failure is experienced at some later time. It will be too late to reconstruct the corrupted data unless it has been previously backed up. Thus, the shock force can cause a catastrophic overwrite in adjacent tracks, thereby obliterating data which had been previously written to the adjacent track. The overwrite results in serious damage to data files which significantly impairs the use of the device.
Protecting the disk from such catastrophic overwrites can only be partially alleviated through mechanical absorption and other known techniques which attempt to insulate the disk drive from such shock forces.
Many disk drives use a write protection method whereby the read/write head(s) is disabled when a position error signal (PES) of the head with respect to the center of the track exceeds some preset amount or threshold. This helps prevent off-track writes. However, during a mechanical shock, the head vibrates and bounces, thereby causing a PES that fluctuates both within and outside the range of acceptable PESs. Thus, for a time period after a shock occurs, due to vibration and bouncing, the head passes over a track and the PES is within an acceptable range. However, the head is merely passing through a track and is really moving between unacceptable PES thresholds and adjacent tracks, and just happens to be within an acceptable PES range at the PES sampling instant. Thus, the PES appears to be acceptable as the head is passing through the track, and the write is ordered. By the time the write is completed executing, however, the head will be in an adjacent track and some of the data will be written onto the adjacent track, because of the movement of the head due to the bouncing, vibrational aftermath of the mechanical shock. Thus, although the PES threshold might not be exceeded at a particular PES sampling instant, the head is passing through the track and moves into an adjacent track and writes in the adjacent track before another PES can be sampled and the write can be turned off, thereby creating a non-recoverable data error. Thus, the prior art merely detects the position of the head and does not anticipate the mechanical bouncing and vibration that can occur during and after a shock.
Many disk drives use shock sensors or accelerometers to detect a mechanical shock. However, the use of shock sensors or accelerometers is disadvantageous because of the additional hardware needed with these devices.
Although the art of disk drives is well developed, there remain some problems inherent in this technology, particularly protecting the data in adjacent tracks during a write operation when a mechanical shock or force is incurred. Therefore, a need exists for a system and method that protects against off-track writing during a disturbance that overcomes the drawbacks of the prior art.